The invention relates to a fabric for use in paper machines.
Various requirements have been placed on paper-making fabrics. For example, avoiding formation of wire marks, improving wear resistance, increasing paper-making retention or improving drainage capability has been required. In particular, increasing the wear resistance of a paper-making fabric has become the most important factor. This is particularly true when considering recent increases in paper-making speed, an increased amount of fillers to be used and an increase in neutral paper-making as well as various measures to reduce manufacturing costs in paper-making companies.
However, the only practical approaches to improving the wear resistance of the fabric are achieved by causing wefts to be worn on the machine side, thereby avoiding wear of warps. This type of the fabric is in a weft wear condition on the machine side. The term "machine side" used herein means the side of the fabric facing the machine. Because the wefts are worn to a greater degree than the warps, some attempts have been made to use, as the wefts, polyamide monofilaments having the desired wear resistance or threads having a larger diameter. However, fabrics having yarns interwoven with the polyamide monofilaments are inferior in stability. To enhance the stability of the fabric, polyester monofilaments may be used. In other words, the polyamide monofilaments can only be used as long as they are interwoven with the polyester monofilament to form the fabric. On the other hand, the threads having larger diameter are not well balanced with other threads and cause an uneven surface of the fabric, resulting in the formation of wire marks. Thus, these methods are disadvantageous from the consideration of practical applications.
Another approach to improve the wear resistance of the fabric has been proposed, for example, in Japanese Laid-open Patent Application No. 62-276097/1987, which discloses a fabric improved in wear resistance. The fabric disclosed is a 14-shaft double layer woven fabric forming a long weft crimp on the machine side of the fabric that is extended under eleven warps. As seen from the paper side of the fabric, this 14 shaft double layer weave fabric looks like a seven shaft double layer weave fabric which has been used for general use. The term "paper side" used herein means the side of the fabric facing the paper to be made, and the term "shaft with a plural number" means the least number of wefts or warps which form a minimum repeating unit of a fabric.
The long crimp of a weft mentioned above increases the effective wearing volume of the fabric and permits the use of yarns having larger diameter, which is advantageous in view of improving the wear resistance.
However, this fabric still has a serious potential problem of forming wire marks.
To facilitate an understanding of the invention, such problems will be described according to FIGS. 7a, 7b, 8a and 8b. When a warp passes over or under a pair of upper and lower wefts, namely, upper wefts being extended on the paper-making side of a fabric and lower wefts extended on the machine side of the fabric, this condition is referred to herein as the wefts are "woven into a fabric". In the case where a warp passes under a lower weft, this is referred to herein as the lower weft interwoven with a warp. On the other hand, another case where a warp passes above adjacent upper wefts is referred to as an upper weft interwoven with a warp.
FIG. 7b is a cross sectional view of a repeating design unit of a conventional seven shaft weave fabric for use in paper machines shown in FIG. 7a, taken along a warp 17 thereof. In FIGS. 7a and 7b, the distance between lower weft 1 interwoven with warp 17 and upper weft 5' interwoven with the warp 17 is equal to three weft counts. On the other hand, the distance between upper weft 6' interwoven with warp 17 and lower weft 1 of the subsequent design is equal to one weft count.
FIG. 8b is a cross sectional view of a repeated unit design of another conventional 14-shaft weave fabric shown in FIG. 8a and has been disclosed in the Japanese Patent Laid-open Application No. 62-276097/1987, taken along a warp thereof 17 of FIG. 8a.
In FIGS. 8a and 8b, wefts 1, 2, 3 to 14 represent lower wefts; wefts 1', 2', 3' to 14' represent upper wefts. In this repeated unit design, the lower wefts 1 and 7 are respectively interwoven with the warp 17, and the upper wefts 4' and 5' 11' and 12' are interwoven with the warp 17.
A cross point between a warp and a weft is referred to as a knuckle. The position of a warp placed on the same side of wefts across two knuckles is referred to as a crimp. For example, a knuckle 19 is formed at the point 1 of lowest weft, and a crimp 18 is formed across the knuckles of interweaving points of upper wefts 4' and 5' (hereinbelow, knuckles will be expressed such as a knuckle 4' and knuckle 5' . . . ).
The magnitude of the force, by which a knuckle of a warp on the paper side is withdrawn downward to the machine side, is in reverse proportion to the distance between the knuckle and the subsequent knuckle interwoven with a lower weft.
As is apparent from FIG. 8b, there is a distance of two weft counts between the crimp 18 formed by upper wefts 4' and 5' and the knuckle 1, and there is a distance of one weft count between same crimp 18 and the knuckle 7. Further, there is a distance of three weft counts between the knuckle 7 and the crimp 18 formed by upper wefts 11' and 12', and there is a distance of two weft counts between the same crimp 18 and the knuckle 1 of the subsequent design unit.
Accordingly, three types of knuckles different in distance between the knuckles and crimps are formed in a repeating design unit. For this reason, since each of crimps of the warp is withdrawn downward from the paper side to the machine side by the different force, the withdrawn or bending angles of the warp is also varied, so that the elevation or level and the shape of each of the knuckles varies, resulting in the difference of the shape of each of the crimps. That is, two kinds of crimps different in height are formed, namely, the crimp between upper wefts 4' and 5' and the crimp between upper wefts 11' and 12' are different in height. The closer the positions of the warp interwoven with a pair of wefts are, the steeper the warp turns. Thus, the paper side of the fabric becomes uneven and the wire marks are formed on the paper made by the fabric. As a result, the fabric disclosed in the Japanese Patent Laid-open Application No. 62-276097/1987 is disadvantageous in that it is useful only to make paper not being required to have smooth surfaces in which the wire marks have no effect on the quality thereof, so that the problem of the wear is not completely solved in a practical way.
As mentioned above, although various approaches to improve the wear resistance of the fabric have been proposed, none of them are sufficient to achieve a satisfactory level of wear resistance in a paper-making environment.